A read-only memory (ROM) is a class of storage used in computers and other electronic devices. Once original data is stored in a ROM, during an execution of the program, it is only allowed to read out the stored information, but not allowed to arbitrarily write in new information or change the original data. Therefore, the ROM is mainly used to distribute firmware, such as fixed programs, constants, character fonts, and operating systems, etc.
Mask read-only memory (M-ROM) is a class of read-only memory. One type of M-ROM uses metal-oxide-semiconductor field-effect transistors (MOSFET) as memory cells. In a MOSFET based M-ROM, impurities are doped into selected channel regions so as to provide the associated memory cells with different threshold voltage levels representing the storage of different values of the binary-coded data. Each ROM cell is in a logic state of “1” or “0” depending on whether ions are implanted into the channel region or not.
However, the MOSFET based M-ROM is limited due to the large cell size and high product cost. In one aspect, size shrinkage of each ROM cell faces big challenges. In another aspect, conventional metal-oxide-semiconductor (MOS) switches suffer a number of problems including, short channel effect, hot carriers, punch through, etc. These disadvantages constrain the applications of MOS switches in further high density M-ROM.
To solve the above mentioned problems, another type of M-ROM uses PN junction diodes as memory cells. In a diode based M-ROM, multiple bit lines correspond to each diode in a same word line so as to provide the associated memory cells with different voltage levels (e.g., Vdd and Vss) representing the storage of different values of the binary-coded data. Each ROM cell is in a logic state of “1” or “0” depending on whether a bit line electrically connects to the associated diode or not.
In a fabricating process of the diode based M-ROM, the multiple diodes are usually produced during formation of an interconnect structure after a CMOS device being formed on a semiconductor substrate. Specific steps include:
First, forming contact holes in an interlayer dielectric layer, filling the contact holes with amorphous silicon and performing a recrystallization process to convert the amorphous silicon into polysilicon. The recrystallization process usually requires a long time (around 10 hours) and a high temperature (about 600° C.).
Next, doping the polysilicon to obtain PN junction diodes, and performing an annealing process to activate the doped impurities. The annealing process also requires a long time (around 1 hour) and a high annealing temperature (about 850° C. to 900° C.). During the process of formation of the diode, a high temperature condition for a long time may cause a negative impact to the CMOS devices on the semiconductor substrate, such as changing the electrical properties, damaging the device and so on, which can result in poor device performances and decreasing reliability.
So, for the diode based M-ROM, it is hard to integrate with the traditional CMOS technique due to high thermal budget after CMOS process, especially for 45-nm-node and beyond.
Accordingly, it is desirable to provide a mask read-only memory (M-ROM), and a related fabricating method to solve one or more problems set forth above and to solve other problems in the art.